1. Field of the Invention
The present invention relates to a bridging agent for improving bonding between incompatible component layers of a laminate. The bridging agent comprises an N-substituted melamine, an N-substituted melamine formaldehyde resin, a hybrid N N-substituted melamine formaldehyde resin, a hybrid N-substituted melamine resin, or various mixtures thereof. The N-substituted melamine has at least one radical group that is an ethylenically unsaturated hydrocarbon containing 3 to 4 carbons. The remaining radical groups are selected from a group comprising hydrogen, ethylenically unsaturated hydrocarbons containing 3 to 4 carbons, alkyl, cycloakyl, aryl, alkaryl, and aralkyl radicals. Exemplary hybrid N-substituted melamine resin are an N-substituted melamine and unsaturated polyester resin, a novel N-substituted melamine formaldehyde and unsaturated polyester resin, and an N-substituted melamine and acrylic resin.
More specifically, the present invention is directed to the use of derivatives of N-substituted melamines to improve the interfacial bond between incompatible component layers of a laminate, for example, between a protective top coat, such as a styrenated polyester, and a melamine resin treated sheet. The present invention is further directed to a method of forming a laminate with improved bonding between incompatible component layers which produces a laminate with better performance characteristics than the prior art laminate products.
2. Description of Related Art
High pressure decorative laminates are conventionally produced by stacking and curing under heat and pressure a plurality of layers of paper impregnated with various synthetic thermosetting resins. The typical build-up, from the bottom up, consists of multiple, for example three to eight, core filler sheets made from phenolic resin impregnated kraft paper. Above the core filler sheets is a decorative sheet impregnated with melamine resin. The decorative sheet can be either a solid color or a print. When a print sheet is used, it is necessary to have an overlay sheet impregnated with melamine resin which, in the laminate, is almost transparent and provides some physical and chemical protection for the print sheet. In other build-ups, having either a solid color or a print decorative sheet, a resin top coat is used to provide chemical protection for the laminate. Without a top coat, decorative sheets impregnated with conventional melamine resins do not have good chemical and physical damage resistance. Unsaturated resins, for example, styrenated polyester, are known in the art as good top coat resins that improve chemical and physical resistance. While providing good top coat properties, unsaturated resins do not bond well to some of the other resins that may be used in the laminate, such as formaldehyde-reactive resins. As an example, styrenated polyester does not bond well to melamine-formaldehyde resin. Consequently, the resulting laminate exhibits separation of the component layers as manifested by blistering and, in severe cases, total de-lamination. Poor bonding between component layers is particularly noticeable when the laminate is formed post-production; that is, when an inside bend is made to form the backsplash of a counter-top.
In order to improve the bond between incompatible layers such as polyester and melamine, the prior art discloses interposing an overlay paper layer, or intermediate dry sheet, at the interface of the incompatible layers. During lamination, the incompatible resins remain separated and penetrate opposite sides of the overlay paper layer. While this technique addresses the lack of direct bonding between incompatible resins such as styrenated polyester and melamine resins, it requires precise control of the amounts and flow characteristics of the polyester and melamine resins so that both sides of the overlay paper layer are coated equally. Both the polyester and melamine resin must flow evenly onto opposite sides of the raw layer and cure at the same rate in order for the raw layer to be effective at improving the bond between the incompatible layers. If the melamine resin flows onto the raw layer and cures first, the polyester resin will not be able to penetrate the raw layer, and vice versa. As a result, the interfacial bond is poor. A further disadvantage of this technique is that, at most, a physical bond, rather than a chemical bond, is created.
In view of the above, there remains a need for a technique to improve bonding between incompatible laminate component layers. For example, there remains a need for a technique to improve bonding between laminate top coats, such as styrenated polyester top coats, and melamine resin impregnated decorative sheets. By improving the bond between incompatible layers, the blister resistance and post-forming characteristics of the resulting laminate would be improved. Further, there remains a need for an agent that can be directly applied to the incompatible layers and to an overlay layer, if a raw layer is desired for other reasons.